1. Field of the Invention
The present invention relates to an image processing system and a computer-readable recording medium for recording an image processing program for performing tone conversion to each local area of an image.
2. Description of the Related Art
Recently, a digital still camera and a digital video camera have become popular devices to capture digital image data. These devices include an image processing system, and perform image processing on a captured image in the system. On such a ground, the bit width of an inputted and processed image signal (for example, 10 to 12 bits) in the image processing system is generally set more widely than that of a final output signal (for example, 8 bits) from the image processing system. This setting is for preventing image quality deterioration caused by cancellation of significant digits through the digital signal processing. In this case, it is necessary to convert the bit width of the signal to match the bit width of the output system. Conventionally, such a bit width conversion has been performed by using a fixed (space invariant) tone characteristic to a standard scene.
In another proposed technique, bit width conversion is performed by using the tone characteristic of each scene. For instance, Japanese patent No. 3465226 discloses a method in which an image signal is divided into areas based on texture information; tone conversion is applied independently to each of the divided areas. However, this method requires high computational cost because of the texture analysis and segmentation. Furthermore, this method can not be considered to preserve spatial continuity (not thought to be spatially smooth).
On the other hand, in international patent application publication No. WO 02/089060, disclosing an image processing system, a high computational efficiency method based on local histogram equalization is proposed. However, this method is not adaptable to some regions like face area, thus showing small improvement.